It has been a problem in the art to produce a clear, thermoplastic film which is tough, chemically resistant and which is useful for optically critical purposes such as a polarizer. Biaxially oriented polyester film, while it is clear and tough, has an optic axis plane angle which varies from point to point across the web. This variation of orientation of the optic axis plane precludes the use of biaxially oriented polyester film for most such optically critical applications.
In the production of polarizing sheets by prior art methods, a film of a substrate is laminated to a film of polyvinyl alcohol as shown by U.S. Pat. No. 4,388,375. This laminate may then be passed through an aqueous staining bath containing iodine and one or more iodide salts to tint the polyvinyl alcohol film. Certain other stabilizing treatments are then employed.
The present invention provides an improved polarized sheeting. Such improved sheeting possesses a very low extinction angle, thus making it highly useful for polarizing applications, and at the same time demonstrates increased adhesion to the polyvinyl alcohol coating thereon. It also demonstrates improved thermal dimensional stability, an improved moisture barrier and reduced water and solvent content. This film may be directly iodine stainable without the need for the formation of a laminate.